1. Field of the Invention
The present invention relates generally to electroless plating apparatus for use in processes based upon the autocatalytic plating of various metals such as silver, gold, platinum, nickel, copper, and palladium onto various substrates. More particularly, the present invention relates to an apparatus for use in a process where the amount of oxygen in the electroless plating solution or bath is controlled to provide stable electroless solutions and in which the rate of autocatalytic metal deposition and the characteristics of the resulting metal plating are controlled in part by the oxygen content of the electroless plating solution.
2. Description of the Related Art
Electroless plating is based upon the autocatalytic or spontaneous decomposition of a metal compound in a plating solution to provide deposition and plating of the metal onto a particular substrate which is immersed in the solution. Electroless nickel plating processes and electroless copper plating processes are well known and commercially widely used. Processes for electroless plating of gold, platinum, palladium and silver are also well known.
Electroless silver plating has particular application to the plating of passive microwave components. Passive microwave components are fabricated by machining, casting, dip brazing, electroforming or resin composite methods. The materials used are aluminum, invar (a nickel-iron alloy), and other metal and resin composites. Generally these components are silver electroplated to improve the Quality Factor (Q Factor) of the device, which is a measure of how well the device performs as a microwave cavity.
The electrical performance or Q Factor of passive microwave components is directly related to the geometry of the component, surface smoothness, silver purity, silver deposit density (voids and lattice misfit), and the specific conductance of its energy propagating surfaces. To provide desired electrical performance, the surfaces are typically silver electroplated to a thickness of between 100 to 1000 microinches (2.54 to 25.4 micrometers) depending on the energy frequency design of the component. Unfortunately, conventional electroplating does not deposit silver uniformly onto all the component surfaces. External surfaces rapidly build up a silver layer while internal, recessed surfaces are barely covered. The result can be a detrimental change of critical dimensions in high current density areas and insufficient silver deposit or plating at recessed surfaces. Although the insufficient and irregular silver thickness resulting from electroplating causes the Q Factor to be less than desired, the silver deposit conductance is still quite good and in spite of these difficulties generally provides a better overall Q Factor for the component than if no silver plating is provided.
The problems with non-uniform silver deposit thickness on complex structures resulting from electroplating can be overcome if the deposit is made by electroless or autocatalytic silver deposition. Electroless or autocatalytic silver plating is capable of depositing a silver layer uniformly over any geometry and is especially well suited for plating passive microwave components.
Although electroless silver technology is well established, silver electroless plating has not become a commercial technology because the plating baths tend to spontaneously decompose forming silver type particles throughout the solution. This decomposition causes loosely adherent, very fine silver metal particles to be deposited roughly on the plating surface at some unknown time during plating. The result is an unacceptable microwave silver deposit. This plating bath instability tendency can be decreased, but not eliminated, by adjustment of the solution concentrations. However, these adjustments lower the silver deposition rate to 0-3 micrometers/hour compared to about 10 micrometers/hour for commercial electroless nickel.
Various different plating surface textures are desirable depending upon the particular application for which the plated surface is to be used. For example, a smooth plating deposit is usually desirable in certain critical applications such as microwave waveguide components to decrease signal losses, or in optical devices and bearings to improve performance. In other situations, it is desirable to provide a plating surface which is rough to dendritic to allow adhesion of various plastics or other materials to the plated surface.
Allowed patent application, Ser. No. 662,110, filed on Oct. 18, 1984 and assigned to the present assignee, discloses a process and system wherein the stability of the autocatalytic plating bath is variably controlled to allow desired metal deposition rates during electroless plating while maintaining the stability of the plating solution when the solution is not in contact with the substrate to be plated. The process also provides a convenient and useful means for varying and controlling the characteristics and texture of the plated surface to produce a wide variety of plating surface textures ranging from smooth to rough. The contents of patent application Ser. No. 662,110, filed Oct. 18, 1984, now U.S. Pat. No. 4,550,036 are hereby incorporated by reference.
The above mentioned process was based on the discovery that the stability of the autocatalytic plating solution, the rate of electroless plating and the characteristics of the resulting metal plating layer could be controlled by varying the oxygen content of the plating solution. The latter process is a cyclic process which basically involves continually removing the active plating solution, which is autocatalytic, unstable and capable of electroless plating at a relatively high rate, from the plating zone or bath and exposing the solution to an oxygen-containing gas to increase the oxygen content of the bath to a predetermined level. The resulting high oxygen content plating solution is stable and does not decompose. This stable solution is not only stable, but also passive, i.e., the rate of plating is very low. The passive and stabilized plating solution is then passed through a scrubber, prior to recycling back to the plating zone or bath, to remove a predetermined amount of oxygen from the stablized solution to provide the desired activity or plating rate as it flows through the plating zone.
When the plating bath parameters such as the residence time of solution in the plating zone, solution temperature, and solution composition are kept constant, the above process provides a convenient means to vary and control the activity (i.e., the plating rate) of the plating solution by controlling the oxygen content of the bath. Decomposition of the bath is also reduced because saturation of the plating solution with oxygen when it is not in the plating zone stabilizes the bath and increases its useful life. An additional feature of the process is that the character of the metal electroless plating can be controlled by varying the oxygen content of the solution. When all other plating parameters are the same, a partially deoxygenated plating solution provides a relatively smooth plating finish whereas a relatively rough plating finish is produced by plating with a plating solution that is substantially completely deoxygenated or a solution which is high in oxygen content.